(a) Field of the Invention
The present invention relates to an apparatus and method for calibrating an offset voltage, and a continuous time delta-sigma modulation apparatus including the same.
(b) Description of the Related Art
For high spectral efficiency, a next generation wireless communication system such as 3rd generation partnership project (3GPP) long term evolution (LTE) and worldwide interoperability for microwave access (WiMAX) uses a modulated wideband signal through orthogonal frequency division multiplexing (OFDM). Therefore, development of a high efficiency transmitter for reducing direct current (DC) power consumption is necessary. Accordingly, an effort for maximizing efficiency has been executed through various structures of power amplifiers such as a Doherty power amplifier and an envelope tracking amplifier. Such a power amplifier is characterized in that a non-constant envelope signal is applied to an input of the power amplifier. When a signal having a large peak to average power ratio (PAPR) is applied to an input of the amplifier, non-linearity of the amplifier increases and thus there is a limitation in improving efficiency of the power amplifier.
In order to overcome a drawback of such a power amplifier, a structure that can embody a transmitter using a switch mode power amplifier (SMPA) instead of a general power amplifier has been suggested.
An input to the SMPA is limited to a signal having a constant envelope, and in order to generate such an input signal, methods such as envelope delta-sigma modulation (EDSM) and envelop pulse width modulation (EPWM) have been suggested. Because the SMPA always operates in a saturation area through such a modulation, linearity of a non-constant envelope signal is guaranteed and high switching efficiency is obtained. A power amplifier of a structure including both an SMPA and a modulation apparatus for converting a non-constant envelope signal to a constant envelope signal is referred to as a class-S power amplifier.
A transmission apparatus using an EDSM method mainly uses a polar coordinate conversion modulation method that separates and transmits a phase signal and an envelope signal. The transmitting apparatus uses a low pass delta-sigma modulator (LPDSM). That is, a transmission apparatus using an EDSM method includes an LPDSM, a phase modulator, a mixer, a power amplifier, and a band pass filter. Envelope information of an input signal is output by a signal in which a pulse width is modulated through the LPDSM, and phase information is output through a phase modulator. An envelope signal and a phase signal are combined by a mixer in a back stage of an LPDSM and a phase modulator, and thus an output of the mixer has a sine wave form having 0 or a constant amplitude, and an output of such a mixer drives a power amplifier of a saturation mode. An output of the power amplifier removes out-of-band quantization noise through a band passage filter, and thus a transmission apparatus using an EDSM method obtains a characteristic of a linear amplifier having high power efficiency.
In order to improve an adjacent channel leakage power ratio (ACLR) by increasing a signal-to-quantization noise ratio (SQNR) by operating with a high over-sampling ratio (OSR), a DSM that is used for such a transmission apparatus uses a continuous-time DSM that can use a high clock frequency. Further, in order to improve an ACLR and error vector magnitude (EVM) characteristics by increasing coding efficiency of a transmission apparatus, the DSM has an order of a second or more, and an output level uses a multiple level of 3 or more.
An envelope signal that is applied to an input of a continuous time DSM through spectrum density analysis is a long-term evolution (LTE) signal having a signal band of approximately 20 MHz, and the signal has a characteristic of a form in which a large amount of signals exist around a DC and in which a predetermined portion of signal magnitude is attenuated according to a bandwidth and in which some degree of signal magnitude is maintained even in an out-of-band. In order to process such a signal, a continuous time DSM should be able to accurately process a signal around a DC, and for this purpose, the continuous time DSM should be insensitively designed for DC offset. Even if a DSM that is used for an EDSM method is not exemplified, a continuous time DSM that is used for a specific system should be insensitively designed for offset or be designed to calibrate offset. Offset that is generated by a cause such as a mismatch of a circuit and an external interference source changes according to other temporal operation conditions such as gain, frequency, temperature, and signal fading, and when the offset is not efficiently removed, signal quality is deteriorated, a dynamic range is limited by circuit saturation, and power consumption increases.